1. Field of the Invention
The present invention relates to a control rod for nuclear reactors such as boiling water reactors and also relates to a method for manufacturing such a control rod.
2. Related Art
One example of a conventional control rod having a long life for boiling water reactor (BWR) is shown in FIGS. 30 to 32 with reference numeral of 200 as disclosed in Japanese Unexamined Patent Application Publication No. 63-8594 (hereinafter referred to as Patent Document 1) or a publication by M. Ueda, T. Tanzawa, and R. Yoshioka of “Critical Experiment on a Flux-Trap-Type Hafnium Control Rod for BWR”, Transaction of the American Nuclear Society, vol. 55, p. 616 (1987) (hereinafter referred to as Non-patent Document 1). Each control rod 200 includes four wings 207, each of them including a sheath 201, made of stainless steel (SUS), having a U-shape in cross section.
Namely, FIG. 30 shows a control rod 200 for a nuclear reactor such as boiling water reactor (BWR). The control rod 200 includes, for example, a tie rod 202 connecting a front end structural member 203 to a terminal end structural member 204, the wings 207 radially extending from the tie rod 202, and a plurality of neutron absorbers 210 arranged in parallel to the axis of the tie rod 202. The front end structural member 203 includes guide rollers 203a and a handle 211 located at an end of the front end structural member 203. Each of the wings 207 includes a sheath 201 having an outer end portion with a U-shape in cross section and has cooling holes 209. The neutron absorbers 8 are accommodated in the sheaths 201.
The control rod 200 shown in FIGS. 31 and 32 is a conventional flux trap-type hafnium control rod 200, which is known to have a long life. The flux trap-type hafnium control rod 200 includes the wings 207 including sheaths 201, hafnium plates 205 accommodated in these sheaths 201, and a tie rod 202. The hafnium plates 205 function as neutron absorbers, are made of hafnium or a hafnium alloy, and are disposed in separated sections of each wing 207 that are arranged in parallel to the axis of the wing 207. Each pair of the hafnium plates 205 are opposed to each other. The respective hafnium plates 205 have different thicknesses depending on the amount of neutrons absorbed by the separated sections. This allows the hafnium plates 205 to have a uniform life.
With reference to FIGS. 31 and 32, the hafnium plates 205 are fixed to the inner surfaces of these sheaths 201, which form shells of these wings 207, with fixing pieces 208 by welding. These sheaths 201 are fixed to this tie rod 202 by means of spot welding.
In the flux trap-type hafnium control rod 200, since the fixing pieces 208 are fixed to these sheaths 201 by means of welding, these sheaths 201 are slightly recessed toward the hafnium plates 205 because of welding distortion. This can eliminate spaces between the hafnium plates 205 and these sheaths 201 and can cause these sheaths 201 and the hafnium plates 205 to be tightly fixed to each other. In this case, any space for absorbing a corrosive component is not present between the hafnium plate 205 and these sheath 201 and a large stress may be applied to the sheath 201 because the hafnium plate 205 cannot be displaced from the sheath 201 although the thermal expansion and irradiation growth of the hafnium plate 205 are different from that of the sheath 201. In the flux trap-type hafnium control rod 200, the hafnium plate 205 is fixed to the sheath 201 with the fixing piece 208 by welding as described above. The welded portion receive relatively large load such as scrum load during operation. The fixing of the fixing piece 208 by means of welding can develop residual tensile stress around the welded portion to cause stress corrosion cracking in the sheath 201 located near the welded portion. This leads to a reduction in the life of the flux trap-type hafnium control rod 200 and may threaten the safety of nuclear reactor.
Japanese Unexamined Patent Application Publication No. 9-113664 (hereinafter referred to as Patent Document 2) also discloses a control rod, manufactured by means of welding, for the BWR. However, Patent Document 1 discloses no technique for reducing residual stresses caused by welding.
In the flux trap-type hafnium control rod 200, each pair of the hafnium plates 205, which are opposed to each other, are disposed in one of the sheaths 201 and a distance between each hafnium plate 205 and the corresponding sheath 201 is maintained with the fixing pieces 208. The welding of the sheaths 201 to upper portions of the fixing pieces 208 causes thin portions of the sheaths 201 to be recessed toward the hafnium plates 205 to develop the residual tensile stress in the sheaths 201.
If the flux trap-type hafnium control rod 200 is used in such a state, the residual tensile stress may cause stress corrosion cracking in the sheaths 201 in cooperation with high-temperature water. The distortion of the sheaths 201 due to welding may eliminate spaces between the sheaths 201 and the hafnium plates 205 to cause crevice corrosion. This leads to a reduction in the reliability of the flux trap-type hafnium control rod 200.
Furthermore, the sheath 201 has an aperture fitted over a projecting portion of the narrow tie rod 202 having a cross shape in cross section and also has an inner space containing the pair of hafnium plates 205 that are neutron absorbers. Each of the wings 207 has a leading portion 211 bonded to a front end structural member 203 and a tailing portion bonded to a terminal end structural member 204.
In the control rod 200, the space between the hafnium plates is filled with water in a nuclear reactor. The reactor water moderates neutrons, which are therefore efficiently absorbed by the hafnium plates 205. Therefore, the hafnium plates 205, which are expensive and heavy, can be saved because of the presence of the reactor water between the hafnium plates 205. The space therebetween is called a trap or a trap space.
The hafnium plates 205 are spaced from each other in the axial direction of the control rod 200, which is inserted into or withdrawn or removed from the nuclear reactor, because the amount of hafnium contained in the hafnium plates 205 located closer to the entrance of the nuclear reactor may be small. The hafnium plates 205 are fixed to the sheaths 201 with fixing pieces 208, referred to as space/load-retaining members, disposed therebetween.
No techniques for preventing stress corrosion cracking are disclosed in conventional technical documents. The hafnium plates 205 are spaced from each other in the axial direction of the control rod 200 and have different thicknesses. However, there are problems in that an increase in the number of the hafnium plates 205 leads to an increase in manufacturing cost and the hafnium plates 205 are nonuniform in mechanical strength in the axial direction (that is, the hafnium plates 205 located at lower positions have lower mechanical strength).
The sheaths 201 are located close to the hafnium plates 205 and therefore the control rod 200 is under corrosive conditions because the stainless steel used to make the sheaths 201 has electrochemical properties different from those of hafnium in the hafnium plates 205. Furthermore, the control rod 200 suffers from corrosion because the atmosphere in the nuclear reactor is corrosive.
Japanese Unexamined Patent Application Publication No. 58-147687 (hereinafter referred to as Patent Document 3) discloses a hafnium control rod including no sheath. The hafnium control rod has a structure for solving a problem that hafnium and stainless steel cannot be welded to each other. The hafnium control rod includes a tie rod made of stainless steel. However, no measure against corrosion or no measure against a problem, called blade history, are disclosed in Patent Document 3.
A long-life control rod is mostly inserted in a nuclear reactor in high-power operation. Therefore, portions of fuel assemblies that are adjacent to neutron absorbers have a low neutron flux level and therefore burn slowly. Hence, fissionable content in the fuel assembly portions is relatively large. When the long-life control rod is withdrawn from nuclear reactor, a large amount of energy is generated. This influences on the health of the fuel assemblies.
This problem may be called blade history. The prevention of a reduction in neutron flux is effective in solving this problem and usually reduces the reactivity worth of the long-life control rod, thereby causing a shortage in reactivity worth.
Conventional control rods have been used in commercial reactors to exhibit satisfactory irradiation resistance. However, it has become clear that the conventional control rods are susceptible to stress corrosion cracking and are electrochemically activated. In order to use the conventional control rods in nuclear reactors for a long time, problems caused by a difference in irradiation growth or a difference in thermal expansion need to be solved and the following problem also needs to be solved in such a manner that a reduction in reactivity worth is suppressed, i.e., a problem that fuel assemblies adjacent to the conventional control rods generate a large amount of power when the conventional control rods are removed from the nuclear reactors (that is, a problem that blade history is serious).
Furthermore, it is desired that neutron-absorbing plates are improved in manufacturability, have a uniform structure in the axial direction thereof, and are reduced in manufacturing cost.